Device for the deflection of electron beams



Dec. 7, 1937. E. BRUcHE El AL DEVICE FOR THE DEFLECTION OF ELECTRONBEAMS Filed Aug. 5, 1955 lnOer-wtors. Err-1st Bruche Walter- Henneberg b8. Their Attor-r1y Patented Dec. 7, 1937 UNITED STATES PATENT OFFICEDEVICE FOR THE DEFLECTION 0F ELECTRON BEAMS Ernst Briiche and WalterHenneberg, Berlin- Reinickendorf-Ost,

Germany, assignors to General Electric Company, a corporation of a NewYork 5 Claims.

The present invention relates to an improved electron deflecting deviceof the type whose operation depends on the combined action of superposedelectrostatic and electromagnetic fields.

For the deflection of electron beams as, for instance, in the so-calledBraun tube or cathode ray tube, or in similar electron optical systems,it has been proposed to use pairs of deflecting windings or deflectingplates. Such devices as have previously been used, however, have theserious disadvantage that they focus as Well as deflect the electronbeam. Since such focusing action is in some applications equivalent to adistortion effect, its occurrence may be extremely objectionable.

It is, therefore, an object of the present invention to provide anelectron prism or deflecting system which is capable of changing thedirection of an electron beam without distorting it in this way. To thisend we may in the practice of our invention use superposed fields,either electrostatic or magnetic, of such nature that the divergingaction of one field exactly compensates the focusing action of theother.

The novel features which we consider to be characteristic of ourinvention will be pointed out with particularity in the appended claims.Our invention itself will best be understood by reference to thefollowing description taken in connection with the accompanying drawing,in which Fig. 1 represents an idealized electron defleeting systemuseful in the explanation of our invention, while Fig. 2 is adiagrammatic view showing another aspect of such a system. Fig. 3 is aperspective view illustrating a structure suitable for the practice ofour invention, and Figs. 4 and 5 are appropriate modifications thereof.

Referring particularly to Fig. l we have illustrated an electrostaticcondenser comprising a pair of mutually concentric cylinders l and 2suitably spaced from one another. It will be readily appreciated that ifpotential is impressed between the two cylindrical plates thus formed,an electrostatic field will be produced in the inter-cylinder spacewhich may be defined as having a magnitude E=ToE0/7. In this equation E0designates the value of the electrostatic field at a particular distanceTo between the radii of 1 and 2. The quantity r is a variable definitiveof any radial distance from the cylinder axis which it is desired toconsider, while E is the electrostatic field strength at such radialdistance, being positive when the inner cylinder is positive and viceversa.

We have also illustrated conventionally a uniform magnetic field Hhaving its axis parallel to the axis of the cylinder 2 and aconcentrated or dipole magnetic field source 3 axially disposed withinthe cylinder 2 and defined as producing at any distance 1- from thecylinder axis an inherently non-homogeneous field H equal to M/r where Mrepresents the strength of the dipole 3.

Suppose the field combination just described to be acting on a particlea: in the inter-cylinder space having a mass m and a charge 2 and at adistance r from the axis of cylinder 2 and moving in a planeperpendicular to that axis. It will then be apparent to those skilled inthe art that the fundamental equations of motion of such a particleare:-

In these equations the various quantities are defined as follows:--

w=dp/dt (the angular velocity of the particle at with respect to thecylinder axis) h=(e/mc)H f=roeEo/m ;r=eM/m'c c=3 10 E and e are given inelectrostatic units, M and H in electromagnetic units.

Integration of Equation (2) gives the following:

r w+/L/r T /2=c=const. (3)

This equation may now be used in connection with Equation (1) toeliminate the quantity w and give the radial distance as a function oftime:

d r/dt =2a /r 3 uc/r +c /r h/2r f/r-h r/4 (4) As a limiting condition itis possible to state that the particle a: will travel in a circle (ofradius To) when in particular dT/dt=0 initially and d r/dt =0. Referringto Equation (1) above, this means that 1'owo (u/To +h)1owof/T0#O (5) webeing used to denote the particular value of angular velocity requiredfor the circular trajectory. This equation can be simplified toh=wo(1y-n) (6) by introducing the quantities 2/=J'/ 0 wo ,fl=#/ o wa (7)(8) Equation (5) represents a solution of Equaton (2) for T=Tc, aconstant. To determine the focusing action of the field combination,assume an electron particle of the same velocity as :1: but whose pathmakes a slight angle a with a tangent to the circular path described by(1;. It is then mathematically possible to define the varying distanceof such a particle from the axis of the cylinder 2 as a function of bothr and a. An ap propriate expression of this relationship is it beingunderstood that 1'1, 1'2 etc. are functions of time or of the angulardisplacement P of the particle. Considerable simplification and asufficient degree of mathematical accuracy will result if all terms in abeyond those of the first power be disregarded. With this in mind substitution of the equality T:T0+a7'1 in Equation (4) will be found toresult in the elimination of all terms not involving a. The resultingequa tion, when modified by disregarding all terms containing a to apower higher than the first and using Equation (6) to eliminate itgives:

The solution obtained by integrating Equation (10) is the following:--

The physical significance of this solution may be best appreciated by aninspection of Fig. 2 in which thedotted lines indicated by the numerals6 and I represent two trajectories in a slightly divergent electron beamassumed to start from a slit 4 as from a point source. It is assumedthat this beam is so directed as to have at its center a circularorbitbetween condenser plates I and 2 which represent sectionalized portionsof corresponding cylinders l and 2 of Fig. 1. It is also assumed thatthe electron beam is subjected to an electrostatic field E, a uniformmagnet field H and a non-uniform magnetic field H similar to thoseconsidered in connection with the foregoing discussion. It will beapparent under the conditions illustrated that when, at time to, anyelectron traverses the entrance slit 4, it will be moving at a definitedistance To from the cylinder axis. At such time, therefore, 11, which,as described above, indicates a deviation from the initial radialposition To, should also be equal to zero. By the interpolation of thevarious applicable quantities in Equation (11) it will appear that thiscondition is in fact satisfied.

If, as has been found experimentally to be the case, a combination ofelectrostatic and magnetic fields such as that under considerationactually exerts a focusing action on an electron beam within itsinfluence, it is possible to predict that at several successive times,t1, t2, etc. later than to, all particles of the beam will pass; throughadditional focal points of radial distance 7'0 from the cylinder axis.By inspection of Equation (9) it will appear that the first of thesefocal points, occurring at time t1 and shown at points 8 of Fig. 2 mustsatisfy the equation:

w0( 1 0)= /1 1 +y -3n Under such circumstances n will again be equal tozero which is the physical condition required to be fulfilled at a focalpoint. Since the quantity wo(t-to) is in fact a representation of theangular distance (indicated in Fig. 2 as P) through which the electronbeam has that moved before reaching the focal point 8 it may be statedas a matter of definition that the focusing angle, P, is given by Tocreate a system in which focusing shall be entirely avoided it is nowonly necessary to establish conditions under which the focusing angle Pshall become infinite. Stated in terms of the relationship alreadyderived this requires or correlatively that By a comparison of this lastequation and the previously derived Equation (6) it will be obvious thatwe have succeeded in defining the relationship which must exist betweenthe original angular velocity of an electron beam and the variouselectrostatic and magnetic forces operating upon it, in order to avoidfocusing of the beam. It should be noted that all the quantitiescontained in Equations (6) and (13) may alternatively be expressed asfunctions of the factors E, H, M, e, m, 10, we and (t1to) as originallydefined.

Referring now to Fig. 3 we have shown an electron-beam prism practicallyembodying our invention. An electron beam I0, having a characteristicenergy V, expressed in volts, which it is desired to deflect withoutfocusing is illustrated as passing between a pair of magnetic fieldcoils II and I2 so distributed as to create therebetween a substantiallyhomogeneous field of uniform strength H. Adjacent the coils II and I2and spaced from their common axis by a distance 1'0, We provide amagnetic dipole l3 in the form of a spool-type coil having aconcentrated iron core IS. The coils H and I2 and the dipole I3 areenergized by voltage sources 20 and 2| respectively. I These sources areindicated schematically and may comprise either a direct current sourceof potential such as a battery or a voltage network depending on the useto which the device is to be put. Voltage varying means includingadjustable impedances 22 and 23 may be provided for adjusting themagnitudes of the respective fields. In accordance with the conventionsalready established we may designate the strength of the dipole as M andthe magnitude of the field created by it as M/r 1 being the distancefrom the dipole axis at which such field is measured; As illustrated andas described in connection with Fig. 1, this field is parallel to thehomogeneous field in the region of beam deflection.

It should be apparent in view of the foregoing discussion and mayreadily be verified by experiment that either of the fields H and Hacting alone will tend to produce a definite focusing or divergence ofthe electron beam. In accordance With our invention, however, the twofields may be made of such nature as to cause mutual neutralization ofthese focusing effects. While the necessary adjustment of values may beattained by experimental means,,we prefer to accomplish it by referenceto the physical relationships established by Equations (6) and (13)above. In the particular case under consideration the computation issimplified by the fact that there is no electrostatic field to beconsidered. For this reason the term g appearing in Equation 13 becomesequal to zero, and the quantity n is readily evaluated as With thissimplification it is an easy matter to 'solve for and M as functions ofthe original electron angular velocity L00 and the radial'distance To.Having due reard to the relation 7 in which account is taken of therelation between electrostatic and practical units of electricalmeasurement and T0 is incm, the desired values of M and H are found tobe:

M (in Oersteds cm 1 1 Zz- H/V H (in Oersteds)=2.25 V/z- Since V is alsothe value of the potential impressed between the cathode and anode ofthe electron beam source, the constants of a nonfocusing beam prism arethus evaluated in terms of easily measurable quantities.

Referring to Fig. 4 we have illustrated an embodiment which differsstructurally from that of Fig. 3 in utilizing a horseshoe magnet I6(having an energizing source 25 and adjusting means 26) in place of theideal dipole l3 described in connection with Fig. 3. For most purposesit will be found that the operation of this modification is exactlysimilar to that already described, and that the same formulas may beapplied for determining the proper strength of the magnet 16.

In Fig. 5 we have shown an arrangement in which deflection of theelectron beam Ill is produced by passing the same through thehomogeneous electro-static field existing between a pair of condenserplates I1 and I8 which are energized from a suitable potential source21. A dipole l9, energized by means of a potential source 29 and avariable impedance 30, is arranged at a distance, say To, from the axisof the beam. If the field potential due to the plates I1 and i8 isassumed to be E at the center or axis of the beam, then the potential Eat any chosen distance r from the dipole l9 will be determined by theformula E=roEo/r. A nonhomogeneous magnetic field transverse to theelectron beam and to the electrostatic field will be superimposed on theregion of beam deflection by the action of the dipole I9. The strengthof this field at any point will be M/r where r again represents adistance measured from the axis of the dipole. In order to determinenonfocusing values of E0 and M in terms of the characteristic energy Vof the electron beam and the quantity To, we may once more havereference to Equations (6) and (13) as derived above. It will beobserved that with the modified structure under consideration thequantity h, appearing in Equation (6) is equal to'zero, since it isassumed that no homogeneous magnetic field is present. Simultaneoussolution of the simplified expressions resulting from this circumstanceyield the two pairs of values for M and E0 as follows:

M (in Oerstedscm )=1.48 rah/V or 15.4 r H/V E (in volts per cm.)=1.l2V/r or-7.12 V/r The meaning of the negative sign in the second pair ofvalues is that the electrostatic field is to be reversed in a directionrelative to the fields so far dealt with. In other words, the innerplate II will become negative with respect to the outer plate l8 so thatthe electric field will exert an influence opposed to that of the dipolerather than in coordination with it as was formerly assumed. Under theseconditions the dipole must be of sufiicient strength to onset thecentrifugal action of both the electric field and the electron momentum.

It will thus be seen that by the use of the principles of our inventionas outlined above an elec-- tron-beam prism may be devised utilizingeither electrostatic or'magnetic superposed fields of such relatedvalues that beam deflection may be obtained witho ut attendantdistortion or focusing. The importance of this possibility in connectionwith electron optical systems pertaining to electron microscopy,television and equivalent arts relating to the reproduction ofelectrical images may be fully appreciated by comparison with analogousdevices as utilized in light optics. In effect, our invention providesan electron optical device of a nature substantially similar to theWellknown light refracting prism.

While we have shown a particular embodiment of our invention, it will ofcourse be understood that we do not wish to be limited thereto sincemany modifications in the structure may be made, and we contemplate bythe appended claims to cover all such modifications as fall .within thetrue spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States, is:-

1. A device for deflecting an electron beam having a characteristicenergy V comprising a concentrated magnetic field source spaced fromsaid beam a distance m and having a strength and means additionallyimpressing on said beam a substantially homogeneous magnetic field ofstrength said field source and said means being arranged so that theirfields are substantially parallel to one another and transverse to thebeam in the region of beam deflection.

2. A device for deflecting an electron beam having a characteristicenergy V comprising a concentrated magnetic field source spaced fromsaid beam a distance To for producing a non-homogeneous field transverseto the beam, said field source having a strength and means additionallyimpressing on said beam an electrostatic field transverse to the fieldproduced by said concentrated field source and having a value variablewith radial distance r from said field source according to the formulaE=1.12V/r for substantially avoiding focusing of said beam.

3. The method of accomplishing distorticnless deflection of an electronbeam which comprises producing in a region traversed by said beam asubstantially homogeneous magnetic or electrostatic field adapted todeflect the beam in a desired manner and superimposing on said region anon-homogeneous magnetic field of such nature and magnitude assubstantially to neutralize the focus-changing effects of saidfirst-named field.

4. An electron beam deflecting device comprising means producing in aregion traversed by an electron beam a substantially homogeneousmagnetic field transverse to the beam for deflecting the same, and meansincluding a concentrated magnetic field source displaced from the axisof the beam for producing in the region of beam deflection anon-homogeneous field substantially parallel with the homogeneous field,said nonhomogeneous field being of such magnitude as substantially toneutralize the focusing effects of the homogeneous field. 1 I

5. An electron deflecting device comprising means producing'in a regiontraversed by an:

of the beam for producing in the region of beam deflection a.non-homogeneous magnetic field substantially transverse to theelectrostatic field and to the beam, said magneticfield being of suchnature and magnitude as substantially to neutralize the focusing effectsof the electrostatic field. 1

. I I ERNsTBRtic E.

WALTER HENNEBERG.

